Skip compressing/writing artifacts that are already in the cache.

Compressing things is expensive, so no reason clients should do it if they don't have to and writing the same thing over and over wastes IO capacity on the cache server.

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/39729173 passed

Bugs closed: 760

Reviewed at https://rbcommons.com/s/twitter/r/1272/
Support passthru args in tasks.

Tasks must declare that they take them.

The last task explicitly mentioned on the cmd-line gets them.

Implemented this for pytest.

Testing Done:
Verified that this does what you expect:

./pants goal test.pytest compile tests/python/pants_test/option/ -- -s

CI passes.

Reviewed at https://rbcommons.com/s/twitter/r/1320/
update the "Pants Essentials" tech talk slides

Nothing changed much; everything changed some

Testing Done:
eyeball

Reviewed at https://rbcommons.com/s/twitter/r/1314/